Conventional RAMs, such as the common semiconductor dynamic random access memories or DRAMs are volatile and not radiation hard, that is, they are corruptible by radiation. This limits the usefulness of DRAMs on spacecraft.
Magneto-resistive random access memories or MRAMs have been developed for spacecraft use to overcome problems associated with volatility and radiation hardness in such applications. Known MRAMs, such as the nickel iron (NiFe) based MRAMs proposed by A. V. Pohm, et al., in "The design of a one megabit non-volatile M-R memory chip using 1.5.times.5 .mu.m cells", IEEE Trans. Magn., Vol. 24, p. 3117, 1988, are both non-volatile and radiation hard, but are limited in application because they have long read access times, on the order of microseconds.
In Pohm's MRAM, the storage element is a ferromagnetic thin film, such as NiFeCo, which is also magneto-resistive. This uniaxial thin film element is used for both data storage and sensing. In operation, the selected direction of magnetization of the thin film is used to represent a binary 1 or 0. As noted, the film is magneto-resistive, that is, the resistance of the film changes when the angle between the applied current and the film's magnetization changes, thus producing a detectable voltage change.
For NiFe based materials, the signal voltage is about 0.5% of the voltage drop across the magneto-resistive element. In the MRAM application, this voltage drop is relatively small because both the current and resistance are small. Thus the read-back signal voltage is relatively small, on the order of 0.45 mV and is in the presence of significant noise, such as thermal noise.
Such known MRAMs have poor signal to noise ratios (SNRs) and require elaborate schemes to accommodate the low SNR and small signals. The resultant read times are unacceptably long, and available techniques for read time improvements are dependent on increasing the physical length of the magneto-resistive elements, reducing storage density.
What is needed is a technique for constructing high speed, high density, non-volatile and radiation hard memory chips.